Method of controlling infiltration of complex-shaped ceramic-metal composite articles and the products produced thereby

ABSTRACT

A process for preparing complex-shaped, ceramic-metal composite articles, comprising: 
     a) contacting a non-wettable powder that is non-wetting to a metal to be used for infiltration with a shaped ceramic body to form a layer(s) of the non-wettable powder on one or more surface(s) of the shaped ceramic body wherein the shaped ceramic body has a region(s) where there is no layer of the non-wettable powder; 
     b) infiltrating the shaped ceramic body with the metal through the region(s) where there is no layer of the non-wettable powder such that a complex-shaped ceramic-metal composite comprising one or more metal phases and one or more ceramic phases is formed, wherein the article has substantially the net shape of the shaped ceramic body and the undesirable regions of excess metal on the surface and undesirable phases within the complex-shaped ceramic-metal composite article near the surface are located only in the region(s) where there is no layer of the non-wettable powder. 
     A complex-shaped ceramic-metal composite article with undesirable regions of excess metal and undesirable phases on the surface(s) of or within the article only where there is or was no layer of non-wettable powder. 
     The process of the invention allows the preparation of complex-shaped ceramic-metal composite articles with undesirable regions of excess metal and undesirable phases on the surface(s) of or within the article only in the regions where there is or was no layer of non-wettable powder. The process of the invention allows the preparation of a complex-shaped ceramic-metal composite article which requires little or no machining of the surface(s) to achieve a finished article. A complex-shaped ceramic-metal composite article is prepared which contains few undesirable regions of excess metal and undesirable phases.

BACKGROUND OF THE INVENTION

This invention relates to a process for controlling the infiltration ofcomplex-shaped ceramic-metal composite articles and the productsproduced thereby.

Ceramics are typically known as low-density materials with high hardnessand stiffness; however, their brittleness limits their usefulness.Furthermore, ceramics are typically formed by creating a densifiedcompact that requires significant and expensive grinding to achieve afinal shape due to the large amount of shrinkage that occurs duringdensification of the compact. Metals are typically non-brittle,non-breakable materials; however, they lack some of the desirableproperties of the ceramics, such as high hardness and stiffness.Therefore, combining a ceramic with a metal can create a compositematerial that exhibits the properties of a ceramic and a metal.

Processes for making ceramic-metal composite articles using ceramicpreforms are known to those skilled in the art. U.S. Pat. No. 5,308,422discloses a process for making ceramic-metal composite articlesinvolving forming layers of ceramic material, sintering the layers ofceramic material into a porous ceramic compact and then infiltrating theporous compact with a metal by immersing the porous body in a bath ofmolten metal. This process involves the uncontrolled infiltration of themetal into the ceramic compact which leads to increased finishing costsdue to the regions of undesirable excess metal and phases formed on thesurface(s) of the composite. An undesirable phase is a reaction phase ofthe chosen ceramic and metal which occurs at the infiltration interface.The reaction phase is chemically unstable or it can cause pullout damageto the surface of the infiltrated part upon machining. Pullout damageresults from machining of the undesirable phase on the surface of thearticle with partial removal of the undesirable phase occurring whichleads to pitting and defects in the surface of the article.

What is needed is a process for preparing complex-shaped ceramic-metalcomposite articles that require little or no finishing of the articleafter infiltration. What is needed is a process for controlling theinfiltration of the metal into the ceramic body, such that the metal isrestricted to certain regions within the article. What is needed is aprocess for preparing ceramic-metal composite article wherein theundesirable regions of excess metal and undesirable phases on thesurface(s) are limited and controlled. What is needed is a ceramic-metalcomposite article wherein the undesirable regions of excess metal andundesirable phases on the surface(s) are limited and controlled.

SUMMARY OF THE INVENTION

The invention is a process for preparing complex-shaped ceramic-metalcomposite articles, comprising:

a) contacting a non-wettable powder that is non-wetting to a metal to beused for infiltration with a shaped ceramic body to form a layer(s) ofthe non-wettable powder on one or more surface(s) of the shaped ceramicbody wherein the shaped ceramic body has a region(s) where there is nolayer of the non-wettable powder;

b) infiltrating the shaped ceramic body with the metal through theregion(s) where there is no layer of the non-wettable powder such that acomplex-shaped ceramic-metal composite comprising one or more metalphases and one or more ceramic phases is formed, wherein the article hassubstantially the net shape of the shaped ceramic body and theundesirable regions of excess metal on the surface and undesirablephases within the complex-shaped ceramic-metal composite article nearthe surface are located only in the region(s) where there is no layer ofthe non-wettable powder.

The invention is also a complex-shaped ceramic-metal composite articlewith undesirable regions of excess metal and undesirable phases on thesurface(s) of or within the article only where there is or was no layerof the non-wettable powder.

The process of the invention allows the preparation of complex-shapedceramic-metal composite articles with undesirable regions of excessmetal and undesirable phases on the surface(s) of or within the articleonly in the regions where there is or was no layer of non-wettablepowder. The process of the invention allows the preparation of acomplex-shaped ceramic-metal composite article which requires little orno machining of the surface(s) to achieve a finished article. Acomplex-shaped ceramic-metal composite article is prepared whichcontains few undesirable regions of excess metal and undesirable phases.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is used to prepare ceramic-metal compositearticles of complex shape comprising one or more metal phases and one ormore ceramic phases. The ceramic and metal are chosen such that themetal will wet and infiltrate the ceramic to form a ceramic-metalcomposite of varying phases. Furthermore, the non-wettable powder ischosen to prevent unwanted interaction between the ceramic and metal.The non-wettable powder limits the point of interface between the metaland ceramic thereby preventing or limiting the formation of undesirableexcess surface metal and phases. Therefore, the phases in the coated andreacted ceramic-metal system are controlled and desirable.

The complex-shaped ceramic-metal composite articles of the invention arearticles with undesirable regions of excess metal and phases on thesurface(s) of or within the article only in the areas where there is nolayer of non-wettable powder. For example, if a shaped ceramic body islayered in non-wettable powder with a small part of the surface areaunlayered, the infiltration of metal will occur through the unlayeredportion. Once infiltration is finished, the unlayered portion of thesurface will be the only part of the surface of the article to containexcess metal and undesirable phases. This greatly reduces the finalmachining and finishing costs since only the unlayered portion needs tobe machined, versus a large fraction of the entire surface(s) of thearticle if conventional infiltration techniques are used. Also, aportion of the surface of the article may be unlayered and thus containundesirable excess metal and phases but not need machining because thatportion is not necessary to the usage of the article. Furthermore, bycontrolling infiltration and the areas of excess metal, undesirablephase formation can be controlled. By controlling undesirable phaseformation, the stability of the phases is controlled along withmachining costs. For example, in aluminum metal systems with carboncontaining ceramics, uncontrolled infiltration leads to the formation ofaluminum carbide and Al₄ BC at the aluminum-boron-carbide infiltrationinterface. Aluminum carbide on the surface of a ceramic-metal compositearticle will react with moisture in the atmosphere and cause corrosionof the surface of the article. Corrosion of the surface leads to anunfinished, rough surface which is undesirable for most applications.Furthermore, undesirable phases can be harder and more difficult tomachine to a given smoothness than the surrounding desirable phases atthe surface of an article. Therefore, when machining is performed on thearticle, the undesirable phases at surface of the article can break offand cause pullout damage leaving unwanted pits and craters in thesurface of the machined article thus making the article unusable for itsintended purpose.

The process can be used to prepare any shape article for whichinfiltration is desired. Preferably, the process is particularlyeffective to prepare thin ceramic-metal composite articles of complexshape. The complex-shaped ceramic-metal composite articles preferablycomprise at least three phases. Preferably, each of the phases ispresent in an amount of at least about 2 volume percent based on thevolume of the multi-phased ceramic-metal material. The ceramic-metalcomposite article preferably has a residual free metal content of about2 volume percent or greater. The ceramic-metal composite articlepreferably has a residual free metal content of about 75 volume percentor less, more preferably about 50 volume percent or less, and even morepreferably about 25 volume percent or less.

The process of the invention may be utilized to produce ceramic-metalcomposite articles in which the metal infiltrates and essentially fillsthe pores of the porous ceramic. Preferably, the ceramic-metal compositearticle has a theoretical density of about 85 percent or greater, morepreferably about 98 percent or greater and most preferably about 99.5percent or greater, wherein the theoretical density (in percent) is 100times the ratio of the final measured part density over the theoreticaldensity of the material with no porosity. The ceramic-metal compositearticles preferably have an elastic modulus high enough to prevent orreduce warping, sagging, fluttering or resonating during handling anduse. Preferably, the ceramic-metal composite article demonstrates anelastic modulus of about 100 GPa or greater, more preferably about 150GPa or greater, and even more preferably about 200 GPa or greater.

The ceramic-metal composite articles of the invention preferablydemonstrate flexure strength high enough to impart shock resistance andresistance to damage during handling and usage. The ceramic-metalcomposite articles of the invention preferably demonstrate a flexurestrength of about 250 MPa or greater, more preferably about 350 MPa orgreater, and even more preferably about 450 MPa or greater. Ifelectrical conductivity is a desired property, the ceramic-metalcomposite articles of the invention preferably have a conductivity highenough to prevent a build-up of static electricity. Preferably, if highelectrical conductivity is a desired property, the composite article ofthe invention demonstrates a resistivity of about 10⁻² ohm-cm or less,more preferably about 10⁻⁴ and even more preferably about 10⁻⁵ ohm-cm orless.

The complex-shaped ceramic-metal composite articles of the invention arepreferably computer disk drive components. Preferably, the articles arecomputer hard disks, E-blocks, actuators, sliders, load beams, supportarms, actuator bearings, spacers, clamps, spindles, ball bearings,thrust bearings, journal bearings, base plates, housings or covers. Morepreferably the articles are E-block actuator components and computerhard disks.

The metals useful in this invention are selected based on theircapability of chemically reacting or wetting with a chosen ceramicmaterial at elevated temperatures such that the metal penetrates intothe pores of the ceramic. Selected metals can be taken from Groups 2, 4,5, 6, 8, 9, 10, 13 and 14 using the new notation of the Periodic Tableas published in the Handbook of Chemistry and Physics, CRC Press, NewYork, N.Y. U.S.A. (1995-1996), and alloys thereof. Preferably, metalsfor use herein include silicon, magnesium, aluminum, titanium, vanadium,chromium, iron, copper, nickel, cobalt, tantalum, tungsten, molybdenum,zirconium, niobium or mixtures and alloys thereof. Aluminum and alloysthereof are preferred because they exhibit high toughness, goodelectrical conductivity and machinability and have good wettability witha chosen ceramic, such as boron carbide, for example. Aluminum is bestemployed as an alloy which provides improved stiffness relative to purealuminum. Alloys of aluminum with one or more of Cu, Mg, Si, Mn, Cr, orZn are preferred. Alloys such as Al--Cu, Al--Mg, Al--Si, Al--Mn--Mg andAl--Cu--Mg--Cr--Zn and mixtures thereof are more preferred. Examples ofsuch alloys are 6061™ alloy, 7075™ alloy, and 1350™ alloy, all availablefrom the Aluminum Company of America, Pittsburgh, Pa.

The ceramics useful in this invention are chosen based on their chemicalreactivity with the chosen metal at elevated temperatures so as toincrease the penetration of the metal into the pores of the ceramic.Preferable ceramics for use herein include borides, oxides, carbides,nitrides, suicides or mixtures and combinations thereof. Examples ofcombinations of ceramics include boron carbides, oxynitrides,oxycarbides and carbonitrides or combinations thereof. More preferredceramics are boron carbides, silicon carbides, titanium diborides andsilicon nitrides. Even more preferred ceramics are B₄ C, AlB₁₂, SiB₆,SiB₄ or combinations thereof. A most preferred ceramic material is boroncarbide, because it has a desirably low density and high stiffness,along with excellent wetting characteristics when in contact with aselected metal. The ceramic material may also be mixed with an organicbinder material such as paraffin wax, stearic acid, or ethylene vinylacetate to facilitate processing. The ceramic material used to form theshaped ceramic body is preferably in powder form and typically containsmetal chemically bonded to the boron, oxygen, carbon, nitrogen orsilicon of the ceramic. The powdered ceramics are preferably crystallinematerials having grains that are about 0.1 micrometers (0.1×10⁻³ mm) orgreater. The powdered ceramics are preferably crystalline materialshaving grains that are about 50 micrometers (50×10⁻³ mm) or less, morepreferably about 10 micrometers (5×10⁻³ mm) or less, and even morepreferably about 5 micrometer (1×10⁻³ mm) or less. The crystallineparticles may be in the shape of equiaxed grains, rods, or platelets.

Examples of preferred ceramic-metal combinations for use in formingmulti-phase ceramic-metal composite articles includes: B₄ C/Al, SiC/Al,TiB₂ /AI, SiB_(x) /AI, SiC/Mg, SiC/Mg-Al, SiB_(x) /Ti, TiN/Al, TiC/Al,ZrB₂ /AI, ZrC/Al, AIB₁₂ /AI, AlB₁₂ /Ti, TiN/Ti, TiC/Ti, TiB₂ /B₄ C/Al,SiC/TiB₂ /AI, TiC/Mo/Co, ZrC/ZrB₂ /Zr, TiB₂ /Ni, TiB₂ /Cu, TiC/Mo/Ni,SiC/Mo, TiB₂ /TiC/Al, TiB₂ /TiC/Ti, WC/Co, and WC/Co/Ni. The use of thesubscript "x" represents that the compound can have varyingstoichiometry. More preferred ceramic-metal combinations include: B₄C/Al, SiC/Al, SiB₆ /Al, TiB₂ /Al and SiC/Mg. Most preferably, thematerials forming the complex-shaped ceramic-metal composite article ofthe present invention are chemically reactive systems such asaluminum-boron-carbide. In these chemically reactive systems, the metalcomponent, after infiltration, can be depleted to form ceramic phasesthat modify article properties such as hardness and stiffness. Thealuminum-boron-carbide composite material includes at least oneboron-carbide-containing phase and at least one aluminum-containingphase. Additionally, the phases may be admixed with a filler ceramic.The filler provides material for the finished article that does notadversely affect the desired properties of the ceramic-metal compositearticle. Filler can be selected from the group consisting of borides,carbides, nitrides, oxides, suicides, and mixtures and combinationsthereof. The filler ceramic is preferably employed in an amount fromabout 1 to about 50 volume percent, based on the volume of themulti-phase ceramic-based material.

The aluminum-boron-carbide composite article preferably includes thephases of B₄ C, AIB₂₄ C₄, Al₃₋₄ BC, AlB₂, AIB₁₂, AlB₁₂ C₂, Al₄ B₁₋₃ C₄The most preferred material is a multi-phase material made of B₄ C, Al,and at least three other ceramic phases, preferably, AlB₂₄ C arepreferably surrounded by aluminum boride and aluminum-boron-carbide. Inother words, the composite article has a continuous ceramic network ofaluminum boron, boron carbide, and aluminum-boron-carbide.

The non-wettable powders useful in this invention are chosen based ontheir ability to coat the article and to prevent undesirable interactionbetween the chosen ceramic and the chosen metal. The non-wettable powderchosen depends upon the particular ceramic and metal system desired tobe infiltrated. The non-wettable powder is chosen so as to benon-reactive and non-wettable to the chosen metal when the metal isinfiltrated into the ceramic substrate. The non-wetting powder preventsthe formation of undesirable excess metal and phases at the surface ofthe ceramic article where infiltration is not desired. However, thenon-wetting powder does not prevent infiltration from occurringinternally to the article once the metal is contacted with some uncoatedpoint on the surface(s) of the ceramic substrate. Examples of systems ofmetals, ceramics and non-wettable powders are aluminum, boron-carbideand aluminum nitride, respectively and aluminum, boron-carbide andaluminum oxide respectively. Preferred non-wettable powders arenitrides, suicides, oxides and combinations thereof. More preferrednon-wettable powders are aluminum nitride, boron nitride, aluminum oxideand combinations thereof. Most preferred non-wettable powders arealuminum nitride, aluminum oxide, and combinations thereof.

Preferably, the powder particles can be any broad distribution of sizeswhich permit the use of the powder as a non-wetting powder on thesurface of a ceramic body. The size of the non-wettable powder particlesis sufficient to prevent infiltration from occurring through the layerof powder particles on the surface of the ceramic substrate. Preferably,the non-wettable powder particles have a particle size of about 0.1micrometers or greater, more preferably about 0.5 micrometer and evenmore preferably about 1 micrometer. Preferably, the non-wettable powderparticle have a particle size of about 50 micrometers or less, morepreferably about 25 micrometers or less and even more preferably about 5micrometers or less.

The ceramic and metal used for the process of the invention depend uponthe product desired. In any embodiment, the metal and ceramic must beselected so as to facilitate infiltration. Infiltration is the processby which a metal, upon melting, forms a solid-liquid interface with aceramic, with the metal as the liquid and the ceramic as the solid, andthe metal moves into the pores of the ceramic material by capillaryaction. The wetting contact angle, as defined by Young's Equation, atwhich infiltration occurs is preferably less than about 90 degrees, morepreferably less than about 45 degrees, and most preferably less thanabout 30 degrees.

The process of the invention involves a series of steps to be performedin order to coat and infiltrate the shaped ceramic body to achieve aceramic-metal composite article of complex shape comprising one or moremetal and one or more ceramic.

The preparation of the ceramic body involves forming a selected ceramicinto a desired article shape. This step can be accomplished by a varietyof ceramic forming processes as discussed hereinafter. The first step ofthe process of the invention involves contacting the chosen non-wettablepowder with one or more surface(s) of the shaped ceramic body. Thenon-wettable powder can be contacted with the shaped ceramic body by anymeans which results in the formation of a layer of the non-wettablepowder on one or more surface(s) of the shaped ceramic body where it isdesired to not have excess metal deposited. Once the non-wettable powderis contacted with the shaped ceramic body, the next step involvesinfiltrating the metal in the ceramic of the shaped ceramic body througha process of heating the metal until it is molten, wherein the metalselectively penetrates the uncoated pores of the ceramic. The metal isinfiltrated into the shaped ceramic body where the non-wettable powderwas not layered onto the body. If desired, after infiltration, a heattreatment may be performed to impart certain other mechanical propertiesto the final complex-shaped ceramic-metal composite article.

In preparing the ceramic body, the selected ceramic is formed into thenear net finished article shape. Any green ceramic-forming process orprocesses may be used which allows the formation of complex-shaped partsat or near net size and shape. Such ceramic-forming processes are wellknown in the art, for example, injection molding, slip casting, tapecasting, gel casting, pressure slip casting, green machining, extrusionand roll compaction. Modern Ceramic Engineering Properties, Processingand Use in Design, D. W. Richerson and Marcel Dekker, Inc., N.Y. 1982.Preferred ceramic-forming processes include injection molding, tapecasting and green machining. The first step of the process of theinvention involves contacting the non-wettable powder with the shapedceramic body in order to form a layer of the non-wettable powder on oneor more surface(s) of the shaped ceramic body. The non-wettable powdercan be contacted with the shaped ceramic body by any means which resultsin the formation of a layer of the non-wettable powder on one or moresurface(s) of the shaped metal body such as thermal spraying (e.g.plasma spraying), atomized liquid spraying, dipping, spinning, brushing,rolling, padding, screening (e.g. screen printing), sol gel coating,electrostatic spraying, electrophoretic depositing, casting (e.g. tapecasting) and combinations thereof. See, for example, Principles ofCeramic Processing, James Reed, 1988 or Handbook of Tribology.Materials, Coatings, and Surface Treatments, supra, relevant parts ofeach incorporated herein by reference. The layer can be a continuouslayer, or a layer can be deposited in a pattern on the ceramic body.Patterns may be formed by a screen printing or a masking technique. Morethan one non-wettable powder can be used at the same time. Multiplelayers of the non-wettable powders can also be used. Furthermore, acombination of non-wettable powders with powders that are wettable bythe chosen metal can be used.

Preferably, the non-wettable powder is blended with a solvent into aslurry mixture in order to improve its ability to be contacted with thesurface(s) of the shaped ceramic body. This can be accomplished by anyconventional technique, such as wet milling. The non-wettable powderslurry comprises the non-wettable powder, a liquid solvent andoptionally one or more of a binder, plasticizer and dispersant.Preferable solvents are water, alcohols and hydrocarbons. The binder canbe any binder which binds the various materials together in the slurrymixture. Preferable binders are wax, resin, gums, polyethylene, latex,acrylics, lanolin, polypropylene, polystyrene, and other thermoplasticpolymers. The plasticizer can be any plasticizer which facilitatesprocessing of the slurry mixture. Preferable plasticizers are glycols,low molecular weight polymers (e.g., liquid at room temperature), oils,fats, and soaps. The dispersant can be any dispersant which promotesdispersion of the non-wettable powder and other materials in the slurrymixture. Preferably, for the dispersant to function efficiently, oneportion of the dispersant needs to be adsorbed on to the particlesurface while the other portion is stretched into the solvent.Generally, the strong adsorption is achieved by acid-based interactionbetween particle surface and the dispersant. Cationic dispersants, suchas amines, are preferred for the negative particle surface and anionicdispersant, such as carboxylic acids, for positive particle surfaces.Dispersants useful in this invention are nonionic dispersants such asethoxylated nonylphenol, anionic dispersants such as magnesium stearate,cationic dispersants such as dodecylamine hydrochloride and ampholyticdispersants such as dodecyl betaine. After milling the non-wettingslurry, it is heated, filtered and de-aired to remove bubbles andagglomerates. The non-wetting slurry is then contacted with one or moresurface(s) of the shaped ceramic body.

Preferably, the non-wettable powder can be contacted with the shapedceramic body using dipping, spraying or brushing. Spraying typicallyinvolves using an atomizer with a spray chamber having an inertatmosphere or in air. After the non-wettable powder slurry previouslydescribed is atomized during the spray deposition process, it is evenlydeposited on one or more surface(s) of the shaped ceramic body. Sprayinginvolves the controlled atomization of a slurry and the directed flow ofthe atomized droplets onto one or more surface(s) of the shaped ceramicbody. On impact with the surface(s) of the shaped ceramic body, thedroplets deform and coalesce into a thick layer. The slurry is driedslowly to prevent cracking of the non-wetting layer, and the dryingtemperature is controlled below the flash point of the chosen solventsystem. The time of drying varies depending upon the solvent used andthe thickness of the layer of the non-wettable powder on the shapedceramic body. It may be necessary to debinder the non-wettable powdermaterial, which can be done by any conventional debindering technique,for example, by heating under a vacuum or in an inert atmosphere.

The layer thickness generally is any thickness which is sufficient toprovide a uniform layer on the surface(s) of the shaped ceramic bodysuch that a complete contacting between the non-wettable powder and theselected ceramic is achieved. The layer thickness is dependent on theamount of non-wettable powder and layer porosity. The thickness of thesprayed layer is dependent on the spray geometry, solids content of theslurry, working distance, spraying time or sequence, rebound loss, andfilm flow. Spraying generally results in uniformity of the layering ofthe non-wettable powder upon the ceramic. The preferred layer thicknessis about 1 particle diameter or greater, more preferably about 10particle diameters or greater, and even more preferably about 25particle diameters or greater. The preferred layer thickness is about0.01 mm or greater. The preferred layer thickness is about 2 mm or less,more preferably about I mm or less and even more preferably about 0.25mm or less.

If desired, the use of screen printing could also be used to impart somegeometry or texturing of the non-wettable powder layer on the surface(s)of the shaped ceramic body, thus further defining the geometry of thecomposite body. A printing screen is utilized to impart the desiredceramic pattern upon the shaped ceramic body during screen printing andthe printed image is dried. Screen printing processes are furtherdescribed in greater detail in Kosloff, Screen Printing Techniques,Signs of the Times Publishing Co., Cincinnati, Ohio, 1981, relevantparts incorporated herein by reference.

The next step in the process involves infiltrating the shaped ceramicbody with the chosen metal such that a shaped ceramic-metal compositearticle is formed. Infiltration is the process by which a metal, uponmelting, forms a solid-liquid interface with a ceramic, with the metalas the liquid and the ceramic as the solid, and the metal moves into thepores of the ceramic material by capillary action. This processpreferably forms a fully dense ceramic-metal composite material. Theinfiltration of the metal into the ceramic occurs through the portionsof the surface(s) of the shaped ceramic body where the non-wettablepowder was not applied. Infiltration can be performed by any method thatis known in the industry, for example, U.S. Pat. Nos. 4,702,770 and4,834,938, both incorporated herein by reference. There are manywell-known ways of infiltrating a metal into a ceramic body. Preferredmethods of infiltration are heat infiltration, vacuum infiltration,pressure infiltration, and gravity/heat infiltration. When theinfiltration is performed, the metal wets and permeates the pores of theceramic that is in contact with the shaped metal body. The degree ofwetting measured by the contact angle between the metal and the ceramicmay be controlled by selecting temperature and time of infiltration. Thetemperature of infiltration is dependent upon the chosen metal.Infiltration is preferably performed at a temperature such that themetal is molten but below the temperature at which the metal rapidlyevaporates. The preferred temperature for infiltration of the selectedmetal into the selected ceramic depends on the melting temperature ofthe selected metal. For aluminum, the preferred temperature forinfiltration of the selected metal into the selected ceramic is about1200° C. or less and more preferably from about 1100° C. or less. Forexample, the preferred temperature for infiltration of aluminum into aceramic is from about 750° C. or greater and more preferably about 900°C. or greater.

For each metal, exact temperature and time of infiltration can beestablished by contact angle measurements to determine when wettingconditions are achieved. Infiltration time is dependent on severalfactors, such as packing density, pore radius, void ratio, contactangle, viscosity, surface tension and sample size. Infiltration ispreferably performed until the metal-infiltrated ceramic material issubstantially dense. Preferably, the infiltration time for a metalselected from the preferred class of metals and a ceramic selected fromthe preferred class of ceramics is about 0.1 hour or greater, morepreferably about 0.5 hour or greater, and even more preferably about 1hour or greater. Preferably, the infiltration time for a metal selectedfrom the preferred class of metals and a ceramic selected from thepreferred class of ceramics is about 24 hours or less, more preferablyabout 12 hours or less, and even more preferably about 6 hours or less.For example, the preferred time for infiltration of aluminum into a 1 mmthick layer of boron carbide at 1100° C. is about 10 minutes.Infiltration can be accomplished at atmospheric pressure, subatmosphericpressures or superatmospheric pressures. The infiltration is preferablyperformed in an inert gas, such as argon or nitrogen, or under vacuum.At superatmospheric pressure, the infiltration temperature can belowered. Infiltration is preferably performed until the ceramic-metalcomposite article is densified to greater than about 85 percenttheoretical density, more preferably greater than about 98 percenttheoretical density, and most preferably to greater than 99.5 percenttheoretical density. Upon completion of the infiltration step, a fullyinfiltrated, complex-shaped ceramic-metal composite article is formed.

After infiltration, heat treatment may be optionally performed on theceramic-metal composite article in order to further tailor mechanicalproperties of the article. A preferred method of altering themicrostructure of already infiltrated ceramic-metal composite articlesinvolves post-infiltration heat treatments of the previously infiltratedcomposite articles. The mechanical properties that can be tailoredinclude fracture toughness, fracture strength, and hardness. Thisadditional step of heating the ceramic-metal composite article at aselected temperature for a selected amount of time will decrease theamount of residual free metal and improve the uniformity of themultiphase ceramic-based material. As a result of the post-infiltrationheat treatment, a slow growth of ceramic phases takes place. It isduring this heat treatment that the greatest control over the formationof multi-phases and the above-stated mechanical properties in theceramic-metal composite article is achieved. The temperature at whichthe heat treatment is performed is a temperature at which the residualfree metal will decrease. Furthermore, the temperature at which the heattreatment is performed is the lowest temperature at which chemicalreactions in the solid state are taking place. A preferred method ofaltering the microstiucture of already infiltrated ceramic-metalcomposite articles involves post-heat treatments of already infiltratedcomposite articles at about 650° C. or greater, more preferably about700° C. or greater. The maximum temperature for post-heat treatment isthe melting point of the metal in the ceramic-metal composite article.The time of heat treatment is preferably long enough that the desiredproperties in the ceramic-metal composite article are achieved byaltering the microstructure.

For example, in the case of aluminum-boron-carbide, this additional stepof heat treating is preferably accomplished by heating the infiltratedbody to a temperature of about 660° C. or greater, more preferably about700° C. or greater and even more preferably about 800° C. or greater.Preferably, the heat treatment is accomplished at a temperature of about1500° C. or less, more preferably at about 1200° C. or less and evenmore preferably about 1000° C. or less. The preferable time period forthe heat treatment of aluminum-boron-carbide is from about 1 hour orgreater, more preferably about 25 hours or greater. The heat treatmentmay be performed in air or an inert atmosphere such as nitrogen orargon. Preferably, the heat treatment is performed in air.

After infiltration and optional heat treatment, the infiltrated body iscooled. Optionally, the infiltrated body may be machined and polishedinto a final desired shape. Any undesirable excess metal on thesurface(s) not treated with non-wettable powder may be machined orpolished to remove the metal. It may be desirable to polish theinfiltrated article depending upon the end usage for the infiltratedarticle. For example, if the desired article is a computer hard disk,the surface(s) of the disk should be polished to a substantially uniformaverage roughness value of between about 1 and about 2000 Å.

Either after infiltration or the optional heat treatment step, thenon-wettable powder may be removed from the surface(s) of the article.Removal of the non-wettable powder may be performed by any method knownto one skilled in the art. Examples of methods of removal includesonication in water or other solvents or mechanical scrubbing.Preferably, the method of removal is by sonication in water.

Also, for example, if the desired article is a computer disk, a coatingmay be applied to the disk in order to impart texture to the surface(s)of the composite article. A suitable coating, for example, is anickel-phosphorus coating; however, other types of coatings can be usedsuch as, for example, metals and polymers. If a nickel-phosphoruscoating is used on an article such as a computer hard disk, the currentindustry procedures for manufacturing and utilizing disks may be used.The coating method may be any that provides dense coating, such asatomic deposition, particulate deposition, bulk coating, or surfacemodification. The most typical method of coating is electroplating. Thecoating itself may be further treated to provide a textured surfaceeither over the entire surface or a portion of the surface. The furthertreatment may be accomplished by techniques such as mechanicaltechniques, chemical or optical techniques, electrical techniques, or acombination thereof.

The process for preparing the complex-shaped ceramic-metal compositearticles allows the creation of complex-shaped ceramic-metal compositearticles with little or no undesirable excess metal on the externalsurface(s) of the article after infiltration so that any article wherenet shape is desirable through forming of the shaped ceramic body andsubsequent controlled infiltration may be formed through this process.Preferred products of this invention are computer hard disks and harddisk drive components, wherein the material has a high hardness, a highwear resistance, a high fracture toughness, a high damping capability, alow density, and a high specific stiffness and is electricallyconductive. There are many other applications for complex-shaped ceramicmetal composite articles such as pressure housings, automotive engineparts, brake systems or any part that requires infiltration.

SPECIFIC EMBODIMENTS

The following are included for illustrative purposes only and are notintended to limit the scope of the claims.

EXAMPLE 1

A boron carbide (B₄ C) greenware part is prepared through mixing 328.4grams of ESK F1500 B₄ C powder with 114.78 grams of an organic binder, aplasticizer and a dispersant. These components were mixed at 60 rpm/130°C. in a 260 mL bowl attached to a Haake Rheocord System 40 torquerheometer, resulting in a mixture with a solid volume content of 50.1percent. This formulation was injection molded into a shape occupyingapproximately 9.1 cm³, at a barrel temperature of about 93.3° C., and aninjection rate of approximately 60.1 cm³ /sec. The part was debinderedaccording to the following approximate heating schedule: 7° C./hr-310°C., 10° C./hr-375° C., 20° C/hr-430° C., 7° C./hr-500° C., 30°C./hr-530° C., and cool to room temperature in a nitrogen atmosphereutilizing an alumina (Alcoa A-16 SG) powder bed.

The part was then spray coated with a 10 v/o Aluminum nitride/ 90 v/oethanol mixture (360 mL ethanol/128 gm AIN mix ratio), and allowed todry. The bottom surface was left unsprayed. About 15.2 g 6061™ aluminumalloy plates were placed in contact with the uncoated bottom surface ofthe part. The part was then placed in saffil beds and infiltrated in anAVS furnace under vacuum conditions at 1160° C., held for 2 hours, andcooled rapidly via a furnace water jacket. The resultant part has excesssurface aluminum only on the base of the part where no non-wettablepowder was placed.

EXAMPLE 2

A greenware part was prepared and molded as in Example 1. The part wasthen dipped in an ethanol and Al₂ 0₃ (non-wetting to aluminum) slurrymixture (20.6 v/o Alcoa A-16 Sg (super-ground), 79.4 v/o ethanol); thebottom surface was wiped clean to provide a surface from which toinitiate infiltration and allowed to dry in air. The part was thenplaced in an alumina bed (Alcoa A-16 Sg) as a wicking aid duringdebindering. The following heating schedule was applied: 7° C./hr-100°C., 5° C./hr-310° C., 10° C./hr-37520 C., 20° C./hr-410° C., 7°C./hr-500° C. cooled to room temperature in a nitrogen atmosphere.Following the debindering step, the coating layer remained adherent tothe part surface, leaving the specimen in a condition directly ready forinfiltration as described in Example 1.

What is claimed is:
 1. A process for preparing a complex-shapedceramic-metal composite article, comprising:a) contacting a non-wettingpowder selected from the group consisting of a nitride, oxide, silicideand mixtures thereof with a porous shaped ceramic body, comprised ofboron carbide, silicon carbide or mixtures thereof to provide a layer orlayers on one or more surfaces of the porous shaped ceramic body and toprovide at least one uncovered surface region where there is no layer ofthe non-wetting powder on the porous shaped ceramic body, b) placing theporous shaped body with the contacting non-wetting powder in a vacuumfurnace and c) infiltrating in the vacuum furnace, under vacuum, theporous shaped ceramic body with a metal through one or more of theuncovered surface regions while maintaining the non-wetting powder layeron the one or more surfaces, such that the complex-shaped ceramic-metalcomposite is formed, wherein the composite has substantially the netshape of the porous shaped ceramic body and the surface of the compositefails to have the metal thereon, except at the uncovered surfaceregions.
 2. The process of claim 1 wherein the metal is silicon,magnesium, aluminum, titanium, vanadium, chromium, iron, copper, nickel,cobalt, tantalum, tungsten, molybdenum, zirconium, niobium orcombinations thereof.
 3. The process of claim 1 wherein the metal isaluminum or alloy thereof.
 4. The process of claim 1 wherein thenon-wetting powder is aluminum nitride, aluminum oxide, boron nitride ormixtures thereof.
 5. The process of claim 4 wherein the non-wettingpowder is aluminum nitride.
 6. The process of claim 1 wherein theceramic of the shaped ceramic body is comprised of boron carbide.
 7. Theprocess of claim 1 wherein the metal is aluminum, the ceramic of theshaped ceramic body is boron carbide, the non-wetting powder is AIN andthe surface of the composite fails to have aluminum carbide and Al₄ BCthereon, except at the regions where there is no layer of thenon-wetting powder.
 8. A process for preparing a complex-shapedceramic-metal composite article, comprising:a) contacting a non-wettingpowder selected from the group consisting of a nitride powder with aporous shaped ceramic body comprised of boron carbide, silicon carbideor mixtures thereof to provide a layer or layers, on one or moresurfaces of the porous shaped ceramic body, and to provide at least oneuncovered surface region where there is no layer on the porous shapedceramic body and b) infiltrating the porous shaped ceramic body under anon-oxidizing gas or a vacuum having a pressure of 1 to 20 millitorrswith a metal through one or more of the uncovered surface regions, suchthat the complex-shaped ceramic-metal composite is formed, wherein thecomposite has substantially the net shape of the shaped ceramic body andthe surface of the composite fails to have the metal thereon, except atthe uncovered surface regions.
 9. The process of claim 8 wherein themetal is silicon, magnesium, aluminum, titanium, vanadium, chromium,iron, copper, nickel, cobalt, tantalum, tungsten, molybdenum, zirconium,niobium or combinations thereof.
 10. The process of claim 9 wherein themetal is aluminum or alloy thereof.
 11. The process of claim 8 whereinthe ceramic of the ceramic shaped body is comprised of boron carbide.12. The process of claim 8 wherein the infiltrating step is performedunder the non-oxidizing gas.
 13. The process of claim 10 wherein thenon-wetting powder is AIN and the surface of the composite fails to havealuminum carbide and Al₄ BC thereon, except at the regions where thereis no layer of the non-wetting powder.
 14. The process of claim 12wherein the non-oxidizing gas is comprised of argon or nitrogen.
 15. Theprocess of claim 8 wherein the nitride powder is comprised of a powderselected from the group consisting of AIN, BN and mixtures thereof. 16.The process of claim 15 wherein the nitride powder is AIN.
 17. A processfor preparing a complex-shaped ceramic-metal composite article,comprising:a) contacting a non-wetting powder selected from the groupconsisting of a nitride, oxide, silicide and mixtures thereof with aporous shaped ceramic body comprised of boron carbide to provide a layeror layers on one or more surfaces of the porous shaped ceramic body andto provide at least one uncovered surface region where there is no layeron the porous shaped ceramic body, b) placing the porous shaped bodywith the contacting non-wetting powder in a vacuum furnace and c)infiltrating the porous shaped ceramic body under a vacuum in the vacuumfurnace with a metal selected from the group consisting of aluminum andalloy thereof through one or more of the uncovered surface regions whilemaintaining the non-wetting powder layer on the one or more surfaces,such that the complex-shaped ceramic-metal composite is formed whereinthe composite has substantially the net shape of the porous shapedceramic body and the surface of the composite fails to have the metal,aluminum carbide and Al₄ BC thereon, except at the uncovered surfaceregions.
 18. The process of claim 17 wherein the pressure of the vacuumis 1 to 20 millitorrs.
 19. The process of claim 17 wherein thenon-wetting powder is aluminum nitride, aluminum oxide, boron nitride ormixtures thereof.